It's easier than you expect: if you want any add on, you have to add the price to the reward you've selected. Let's see with an example:
Chris wants to select the $225 reward to get the signed illustration by Emma Ríos. But he wants to get the 120mm miniature too! He needs to select the $225 reward and add the miniature price. This means $225 + $75 = $300
Then, when the Kickstarter campaign has been finished, we'll send you a form to select the add on you want.
Anyway, if you need help with the add ons, you can contact us!